1. Field of the Invention
The present invention relates generally to electrophoretic displays. A matrix driven electrophoretic display with a multi-layer back plane is disclosed.
A plastic display, such as an electrophoretic display, typically comprises a lower electrode layer, a display medium layer, and an upper electrode layer. Biasing voltages typically are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrodes being biased. For example, a typical passive matrix electrophoretic display may comprise an array of electrophoretic cells arranged in rows and columns and sandwiched between a top and bottom electrode layer. The top electrode layer may comprise, for example, a series of transparent column electrodes positioned over the columns of electrophoretic cells and the bottom electrode layer may comprise a series of row electrodes positioned beneath the rows of electrophoretic cells. A passive matrix electrophoretic display is described in Provisional U.S. Patent Application Ser. No. 60/322,635 entitled “An Improved Electrophoretic Display with Gating Electrodes,” filed Sep. 12, 2001, which is hereby incorporated by reference for all purposes.
The design of a passive matrix display, such as a passive matrix electrophoretic display, typically must address the problem of cross bias. Cross bias refers to the bias voltages applied to electrodes that are associated with display cells that are not in the scanning row, i.e., the row then being updated with display data. For example, to change the state of cells in a scanning row in a typical display, bias voltages might be applied to column electrodes in the top electrode layer for those cells to be changed, or to hold cells in their initial state. Such column electrodes are associated with all of the display cells in their column, including the many cells not located in the scanning row.
One known solution to the problem of cross bias is to provide an active matrix display instead of a purely passively matrix display. In an active matrix display, switching elements such as diodes or transistors are used, either alone or in conjunction with other elements, to control pixel electrodes associated with the display cell or cells associated with an individual pixel. In one typical active matrix display configuration, for example, a common potential (e.g., ground potential) may be applied to a common electrode in the top layer and pixel electrodes located in the bottom layer are controlled by associated switching elements to either apply a biasing voltage to the pixel electrode or to isolate the pixel electrode to prevent an electric field from being generated that would cause the associated display cell(s) to change state. In this way, one can control the effect of cross bias by isolating the pixel electrodes associated with display cells in non-scanning rows, for example.
Active matrix displays are known in the art of liquid crystal displays (LCD). One typical design employs thin film transistor (TFT) technology to form switching elements adjacent to the respective pixel electrodes with which they are associated. However, this approach is expensive and time consuming and, as a result, does not scale well to a very large display. Also, a high temperature resistant substrate such as glass is typically used in TFT LCD displays. The TFT substrate is rigid and may not be well suited for applications requiring, for example, a flexible plastic display, which may in some cases be fabricated most efficiently by a roll-to-roll process requiring a flexible substrate.
The TFT-LCD technology may not be suitable for an active matrix electrophoretic display for other reasons. For example, a microcup electrophoretic cell is described in co-pending applications, U.S. patent application Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. patent application Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. patent application Ser. No. 09/606,654, filed on Jun. 28, 2000 and U.S. patent application Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The microcup electrophoretic display described in the referenced applications comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. For such cells, it may be critical to have a nearly even top surface for the bottom electrode layer to ensure adequate sealing upon lamination of the bottom electrode layer, electrophoretic cell layer, and top electrode layer. The TFT technology described above may result in structures too thick for such an application.
A further shortcoming of the typical TFT technology for use in an active matrix EPD is that the switching elements typically are formed adjacent to the respective pixel electrode(s) with which they are associated. The presence of such elements between the respective pixel electrodes may affect resolution adversely by requiring excessive space between pixels.
In a large size active matrix EPD, active switching components may also be in the form of discrete components or one or more integrated circuits. In such a system, there is a need to route conductive traces from the switching components to the driver and/or control circuits and components. The problem of routing connections to electrodes may also be encountered in a passive matrix display in which one or more screen splits have been introduced in the column or row electrodes, for example by splitting a column electrode into two or more segments to improve response time, as each electrode would have to make contact via a conductive trace with a driver configured to provide the prescribed biasing voltage to the electrode.
For either an active matrix or a passive matrix display, it may be advantageous and/or necessary to route signals between an electrode located at a first location in the plane of the display (or along the surface of the display, if not flat) and a switching, driver, and/or control element located at a second location in the plane of the display. One shortcoming of routing conductive traces along the plane of the display between electrodes and associated elements in a single layer is the risk that undesired electric fields will be established, or desired electric field interfered with, by virtue of potentials applied to such traces (i.e., the trace may act as an electrode, potentially affecting the migration of charged particles in one or more electrophoretic cells positioned near the trace). For more complicated designs (e.g., large number of switching elements, large number of pixel electrodes, passive matrix with large number of “splits”, etc.), it may be difficult to lay out in a single layer all of the conductive traces necessary to interconnect the electrodes and associated components, as needed, especially in a manner that does not affect display resolution and performance adversely.
Via structures for use in an electrophoretic display have been described for connecting a conductive structure in one layer to a conductive structure in another. One such structure is described in U.S. Pat. No. 3,668,106 to Ota, issued Jun. 6, 1972, which is incorporated herein by reference for all purposes. One other such structure is described in an article entitled, “An Electrophoretic Matrix Display with External Logic and Driver Directly Assembled to the Panel,” by J. Toyama et al. (SID 1994 Digest, pp. 588-591). However, previously described via structures have typically been used to connect a first conductive structure in one layer (or on one surface of the substrate through which the via structure communicates) to a second conductive structure in another layer (or on another surface of the substrate) that is located immediately beneath (i.e., opposite) the first conductive structure. One other approach is described in U.S. Pat. No. 6,312,304, issued Nov. 6, 2001, which is incorporated herein by reference for all purposes. The structure described in the latter patent comprises three layers: a modulating layer comprising an electrophoretic display media, a pixel layer comprising pixel electrodes which provide the driving voltage to the display media and connect to contact pads on the bottom surface of the pixel layer through vias, and a circuit layer comprising circuit elements. The three layers are laminated together to form a device. Although such a structure has the advantage of allowing each component to be manufactured using processes optimized relatively independently of the requirements and properties of the other components, in practice the approach is limited to devices using thin film circuit technology. It does not address the circuit trace routing issues in a multiple-split passive design nor the driving circuit requirement on a large size display panel.
Therefore, there is a need for a matrix driven electrophoretic display (active and/or passive matrix) made using technology that is relatively inexpensive, does not affect resolution adversely, and is suitable for use with electrophoretic cell designs such as the microcup electrophoretic cell described above. In addition, there is a need for a matrix driven electrophoretic display in which the top surface of the bottom electrode layer is sufficiently even to provide for adequately sealing of the electrophoretic cell layer. Also, there is a need for a matrix driven electrophoretic display technology that is suitable for use with large-scale displays, and for use in a flexible plastic matrix driven electrophoretic display, including displays made using roll-to-roll production technology. Finally, there is a need to provide all of the above in a manner that does not affect display resolution and performance adversely, such as by requiring numerous conductive traces in the same layer as the electrodes to route required signals and/or potentials to the electrodes.